The quad flat no-lead (QFN) component family, as seen in Figure 1, is one of the newest electronic packages to be introduced into PCB design. The QFN has pins on 4 edges of the bottom surface of the package. The QFN can have either a square or rectangle body as well as symmetric or asymmetric terminal patterns. The QFN with symmetric pins is only available in millimeter pin pitches of 0.8 mm, 0.65 mm, 0.5 mm and 0.4 mm as per the standard JEDEC MO-220I.
The QFN was introduced to replace the gull-wing lead Quad Flat Package (QFP) because the component leads are embedded in the plastic and cannot be bent during handling to insure consistent assembly attachment. The embedded lead form is also compatible with high-speed design as the die-to-lead bonding and PCB contact is much shorter. Plus, due to the high-speed aspect, the component generates a lot of heat. This is why most QFN packages have a thermal tab that is via stitched to the GND planes for heat dissipation.
Figure 1. QFN packages.
The QFN package is a CSP (plastic encapsulated package) with a copper lead frame substrate. The package is leadless where electrical contact to the PCB is made by soldering the leads on the bottom surface of the package to the PCB, instead of the conventional formed perimeter gull wing leads. The design of the QFN package has enhanced electrical performance that enables the standard 2 GHz frequency to be increased up to 10 GHz with some design considerations.
The QFN leads are coated with a finish that provides environmental protection and maintains solderability. See Figure 2 for a cross-section internal view of the QFN package construction.